Some of conventional image-forming apparatuses are provided with a paper-punching device for forming holes through sheets of paper in order to improve the efficiency of jobs for sorting the sheets of paper that have been subjected to the image-forming operations into a set of documents. Moreover, in recent years, with the wide spread of office automation apparatuses for handling sheets of paper, such as used in copying machines for ordinary paper (PPC) and automatic paper feeders (APF), and with the trend to high-speed, highly-effective operations in those apparatuses, there have been also strong demands toward a fast, highly-efficient punching operation, which is required before filing sheets of paper that have been subjected to the copying operation.
As one example for such an paper-punching device, Japanese Laid-Out Patent Publication No. 140755/1983 (Tokukaishou 58-140755) has disclosed a paper-punching device. The following description will discuss this paper-punching device. Here, for convenience of explanation, the following example has an arrangement that is slightly different from that of the above-mentioned patent publication.
As illustrated in FIG. 58, in the above-mentioned paper-punching device, a sheet of paper P is transported by transport rollers 303 and 304 from the upstream side on a base 301 while it is restricted in its upward dislocation by a transport guide 302. When the leading edge of the sheet of paper P passes through a light path of a photosensor 305 of the reflection type, the leading edge of the sheet of paper P is detected by the photosensor 305. Then, a stopper 306, located on the downstream side of the photosensor 305, moves upward from its stand-by station, and presses the leading edge of the sheet of paper P, thereby stopping the transportation of the sheet of paper P.
Immediately after the stoppage of rotation of the transport roller 304, a punching blade 307 is shifted down toward a punching die 308 that is provided in the base 301, and the sheet of paper P is thus punched by the punching blade 307. At this time, the transport roller 304, located on the downstream side, is stopped in its rotation, while the transport roller 303, located on the upstream side, is being rotated.
Therefore, the sheet of paper P is transported by the transport roller 303 from the rear-edge side, and is warped inside a warp-space 302a that is provided in an upward-raised form between the transport roller 303 and the transport roller 304 in the transport guide 302. With this arrangement, the transportation of the sheet of paper P is not stopped completely. Further, since the pressing force of the stopper 306 exerted onto the sheet of paper P is increased, the sheet of paper P does not retreat even upon the punching operation.
However, in the paper-punching device as described in the above-mentioned patent publication, the leading edge of the sheet of paper P is temporarily stopped even if it is for a short period of time. Therefore, when the operation speeds of the image-forming process and other related processes are increased beyond a certain limit, the next paper is transported although the proceeding paper has not been subjected to the punching operation; this causes troubles such as paper jams. Moreover, the sheet of paper P might be damaged when it is warped. Furthermore, if the sheet of paper P is thick paper weighing not less than 228 g/m.sup.2, it is not allowed to warp, thereby making it difficult to keep transporting sheets of paper P by the use of the transport roller 303 during the punching operation. Another problem is that it is not possible to form punch holes on the rear side of the sheet of paper P due to the structure of the paper punching device.
Moreover, although not described in detail here, another paper-punching device for use in an electrophotographic apparatus has been known to the art, wherein a punching operation is carried out with a sheet of paper P completely stopped, and after a punching operation the transportation is resumed. In this type of paper-punching device, it is possible to install the punching mechanism either on the leading side or on the rear side of the sheet of paper P. However, since the punching operation is carried out after stopping the sheet of paper completely, it is impossible to increase the speed of the operation.
Another example is Japanese Laid-Out Patent Publication No. 190696/1991 (Tokukaihei 3-190696), which discloses an arrangement wherein a paper-punching device, which has two punching claws that are disposed in the direction orthogonal to the transporting direction of a sheet of paper at positions corresponding to an interval between punch holes, is installed on the paper-discharging side of a copying machine; and sheets of copy paper, which are discharged successively by discharge rollers after having been subjected to the copying operation, are subjected to punching operations that are carried by activating both of the punching claws at the same time by using a cam that operates in synchronism with the discharge rollers that are discharging the sheets of paper.
However, in this arrangement the two punching claws for forming punch holes are disposed at predetermined positions that correspond to the punching positions that are located in the direction orthogonal to the transporting direction of a sheet of paper; this fails to deal with differences in the punch-hole intervals and the number of punch holes. Therefore, the problem of this arrangement is that it is necessary to provide as many punching claws as the number of punch holes and allow these punching claws to have the corresponding punch-hole intervals, thereby making the construction more complicated and expensive. Further, the sheet of paper P has to be temporarily stopped upon forming punch holes; therefore, in the case when the operation speeds of the image-forming process and other related processes are increased, problems such as planar jams might be caused in the same manner as in the paper-punching device disclosed in Japanese Laid-Out Patent Publication No. 140755/1983 (Tokukaishou 58-140755). This makes it difficult to achieve a high-speed operation.
In contrast, for example, Japanese Laid-Out Patent Publication No. 105895/1992 (Tokukaihei 4-105895) discloses another paper-punching device wherein a piezo-electric element that enables an extremely high-speed operation is used as the source of driving force of the punching claw; a sheet edge sensor, which detects the position of the leading edge of a sheet of paper in the transporting direction, is disposed at a position a predetermined distance apart from the punching claw, the distance being equal to a distance from the detected paper edge to the first punch hole in the transporting direction; and a punch-hole sensor is disposed at a position a predetermined distance apart from the punching claw, the distance being equal to an interval between the punch holes. In this paper-punching device, the first punch hole is formed in the sheet of paper by the punching claw in response to the detection of the position of the leading edge of paper made by the sheet-edge sensor; and the second punch hole is formed in the sheet of paper by activating the punching claw in response to the detection of the first punch hole made by the punch-hole sensor, with a predetermined punch-hole interval from the first punch hole.
With this arrangement, for example, in the case of forming two punch holes in a sheet of paper, the leading edge of the transported paper is first detected by the sheet-edge sensor, and in response to the detection, the punching claw is activated once, thereby forming the first punch hole. Thereafter, when the sheet of paper is further transported, the first punch hole is detected by the punch-hole sensor, and in response to the detection, the punching claw is further activated one more time, thereby forming the second punch hole. This arrangement eliminates the necessity of having to install as many punching claws as the number of punch holes and having to align these punching claws with the corresponding punch-hole intervals; thus, one punching claw is allowed to deal with differences in punch-hole intervals and in the number of punch holes, thereby making the device simpler and less expensive; and it becomes possible to form punch holes without the necessity of stopping the transportation of a sheet of paper.
In the arrangement disclosed in Japanese Laid-Out Patent Publication No. 105895/1992 (Tokukaihei 4-105895), the punching claw is fixed to a predetermined position corresponding to the punching position in parallel with the transporting direction of paper. Therefore, as to punching positions along the transporting direction, various changes may be provided by changing the driving timing of the punching device. However, as to punching positions in the direction orthogonal to the transporting direction, setting in desired positions is not allowed because of their fixed state.
For this reason, this paper-punching device is applicable to copying machines and other apparatuses wherein sheets of paper are aligned along one side of the transport path and transported; however, it is not applicable to copying machines and other apparatuses of the so-called center-oriented type, wherein sheets of paper are positioned based on the center of the transport path and transported.
In order to solve this problem, there has been proposed another arrangement which has a plurality of punching claws that are aligned in the direction orthogonal to the transporting direction of paper, for example, at punching positions corresponding to the respective sizes of sheets of paper that are transported on the center basis. However, this arrangement requires individual driving sections for driving the respective punching claws, thereby making the construction more complicated as well as causing high costs because a lot of expensive piezoelectric elements are needed.